Journal of Medical Physics
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Year : 2000  |  Volume : 25  |  Issue : 1  |  Page : 6-12

Measured Head Scatter Correction Factors For Telecobalt Units

Correspondence Address:
SD Sharma

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Source of Support: None, Conflict of Interest: None

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Head scatter correction factor (Sc) is used to account for in-air variation of the output of mega voltage photon beams. The magnitude of Sc depends on the construction and material of source head of the teletherapy unit, depth of measurement, interchange of collimator jaws and introduction of beam' modifying wedges. SC for four telecobalt models, Theratron 1000E, Theratron 780E, Theratron 780C and Theratron Phoenix, were measured to study (a) the effect of contaminating electrons on the value of Sc measured at the depth of dose maximum (dmax); (b) collimator exchange effect on the value of SC; (c) effect of beam modifying wedges on the values of Sc; (d) unit-to-unit variation of Sc for the same model of the telecobalt machine; and (e) variation of Sc with different models of the telecobalt machines of same manufacturer. Results of these measurements indicate that the effect of contaminating electrons on the value of SC measured at dmax, the colimator exchange effect and the effect of wedge filters on the values of Sc are within the experimental uncertainty and can be neglected. However, the variation of Sc with different units of the same model and with different models of the same manufacturer of the telecobalt machine is larger than the experimental uncertainty and hence Sc data or clinical use should be measured with each and every unit of the telecobalt machine.

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